This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-294979, filed Sep. 27, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of producing an exposure mask to be used in manufacturing a semiconductor device, and more specifically, the invention relates to a method of producing an exposure mask such as a phase shift mask and the like which are produced through at least two processes of exposure.
2. Description of the Related Art
For recent years, much attention has been attracted to a phase shift mask as an exposure mask, where resolution is improved by use of a phase difference of light passing through adjacent patterns. Among this type of phase shift mask, in the course of production of a half tone type exposure mask, a phase shift film, a light blocking film and a resist film are formed on a transparent substrate, and then a first exposure (patterning) is carried out thereon, and a resist pattern is formed by development of the resist film. Then, by selectively etching the light blocking film and the phase shift film by use of this resist pattern, a device pattern (main pattern) is formed on a main pattern area. Next, carried out is a second exposure, namely, a superposition exposure for forming an opening pattern around the device pattern that has been formed by the first exposure. The problem in the process of this superposition exposure is the positional relationship between the main pattern and the opening pattern with which the superposition exposure is carried out onto the main pattern.
Conventionally, in order to set the relative positions of the main pattern and the opening pattern, the following method has been employed. For example, in the case of first exposing and forming a main pattern, when the main pattern is exposed onto a resist film on a main pattern formation area, a plurality of alignment marks are exposed, at the same time, onto a resist film on an area other than the main pattern formation area. At this moment, the main pattern and the alignment mark are formed on the same coordinate system. Then, in the case of carrying out the superposition exposure (second exposure) of the opening pattern onto the main pattern already formed, first the plural alignment marks that have been already formed are detected through the resist film coated on the mask substrate prior to the second exposure. The coordinate system is obtained according to the detected alignment marks, and the opening pattern is exposed according to this coordinate system.
By use of the method, the coordinate system of the main pattern and that of the opening pattern may be the same ideally, and there is no positional error. However, in actual conditions, the position where the alignment marks are formed is displaced from an objective or correct position. This is caused because a vacuum chuck is employed so as to hold a mask substrate in an exposure apparatus, and further, the mask substrate is deformed under influence of gravity. The positional error caused by such deformation appears larger in the outside portion than the center portion of the mask substrate. In ordinary cases, the main pattern is formed at the center portion of the substrate, and alignment marks are arranged on the outside portion thereof. As a consequence, the positional error of alignment marks will be larger than that of the main pattern. Therefore, in carrying out the superposition exposure or the second exposure, a displaced alignment mark is referred to, as a result, it has been extremely difficult to restrict the relative positional error between the main pattern and the opening pattern in superposing exposure to 50 nm or less.
On the other hand, a sequential exposure method is employed for forming the opening pattern in the phase shift mask. The sequential exposure method is performed, for example, by using an exposure machine such as CORE-2564PSM which is a product of ETEC Corporation. In this machine, a spot-shaped laser beam is used to scan a pattern formation area on a mask substrate. A resist film is applied in advance onto the substrate, and the property of a portion of the resist film radiated by the laser beam is changed to be soluble to alkali developing solution. By use of this sequential exposure method mentioned above, and by radiating laser beam onto a portion where resist should be removed, and not doing so onto the portion where resist should be left, it is possible to selectively make the portion where resist is removed and that where resist is left intact.
The biggest problem in exposure an opening pattern by use of the sequential exposure method is that the exposure takes much time. In the case of exposure patterns by scanning a laser beam spot, the period of the pattern drawing time is in proportion with a magnitude of an area to be exposed. The opening pattern has a large area, and the exposure time will be long. For this reason, the throughput in producing phase shift masks will be deteriorated inevitably. And further, an exposure apparatus employing a sequential exposure method in general appears with expensive prices, and when this type of apparatus is occupied for long hours for exposure an opening pattern, the manufacture costs of phase shift masks will be increased.
As mentioned above, in the prior art, production of half tone type phase shift masks has required two times of exposure process, and in exposure for opening patterns by a second superposition exposure, it has been important to precisely set the position of the main pattern formed by a first exposure and the position of the opening pattern to be superposed thereonto. In the conventional method, however, positional error of alignment marks is large, therefore it has been very difficult to make small the positional error at superposition exposure, which has been a problem with the prior art. And further, the sequential exposure method has been employed for exposing an opening pattern necessary for manufacturing a phase shift mask, but the sequential exposure method requires long exposure time, thereby leading to deterioration of the throughput.
According to an aspect of the present invention, there is provided a method of producing an exposure mask by carrying out patterning with at least two times of exposure on a mask substrate, comprising: performing a first time exposure of a first pattern and an alignment mark which is used as a reference for positioning at a second time exposure of a second pattern, the alignment mark being exposed onto a corrected position on the mask substrate with positional error by substrate deformation obtained in advance with a first exposure apparatus to be used corrected, and performing a second time exposure of the second pattern, by detecting a position of the alignment mark with a positional error caused by a substrate deformation obtained in advance in a second exposure apparatus to be used corrected, so as to position the second pattern with respect to the first pattern according to the detected position of the alignment mark.